In a process of manufacturing a semiconductor, and a magnetic disc, use is made of a charged particle beam measuring instrument wherein a specimen is irradiated with a charged particle beam (hereinafter referred to as a primary beam), such as an electron beam, an ion beam, and so forth, to acquire a signal of a secondary charged particle (hereinafter referred to as a secondary beam) such as a secondary electron emitted, and so forth, thereby measuring a shape and size of a pattern formed over the specimen, and a charged particle beam inspection apparatus for checking presence of a defect, and so forth.
For such a charged particle beam apparatus as above, use has thus far been made of an apparatus in which scanning with a primary beam converged in a stream is executed over a specimen, the so-called SEM apparatus. The SEM apparatus, however, has had a problem in that much time is required for acquisition of an image since the image is acquired by two-dimensional scanning with the primary beam, so that other techniques have been under studies in order to improve a speed for processing a specimen, that is, an inspection rate.
First, a multi-beam type charged particle beam apparatus using plural beams has been proposed as a first approach. For example, in JP-A-2007-317467, there has been disclosed a multi-beam type electron beam inspection apparatus wherein an electron beam discharged from a single electron gun is divided into plural beams, and the plural beams formed by individually converging with the use of an array of lenses are applied onto a specimen for scanning with the use of a single optical element.
FIG. 1 is a schematic diagram showing an electron optical system of a multi-beam type charged particle beam apparatus that is constructed by disposing an electron source 101, a multi-beams forming unit 102, a objective lens 103, a scanning deflector 104, a specimen 105, secondary electron detectors 106a to 106c, and so forth. A primary beam 107 outgoing from the electron source 101 is turned into plural (in FIG. 1, three) primary beams after passing through the multi-beams forming unit 102, and the plural primary beams are individually focused to thereby form plural electron source images 108a to 108c, respectively. The plural primary beams 107 pass through the objective lens 103, thereby scaling down the plural electron source images 108a to 108c, respectively, to be projected over the specimen 105. The scanning deflector 104 causes multiple beams formed due to the plural primary beams 107 passing through the multi-beams forming unit 102 to undergo a deflection action substantially in the same direction, and substantially by the same angle only, respectively, thereby scanning the specimen 105. The plural primary beams 107 having reached the surface of the specimen 105 react mutually with material present in the vicinity of the surface of the specimen, whereupon electrons of secondary nature such as backscattered electrons, secondary electrons, Auger electrons, and so forth are emitted from the specimen to be thereby turned into plural secondary beams 109, to be detected by the secondary electron detectors 106a to 106c, respectively. Thus, with the multi-beam type charged particle beam apparatus, use of plural beams enables acquisition of information over a specimen at a speed several times as fast as that in the case of the SEM apparatus, and the further the number of the multiple beams is increased, the higher the inspection rate can be enhanced.
Now, with the multi-beam type charged particle beam apparatus, use is made of a single optical element such as, for example, the objective lens 103, so that the further the number of the multiple beams is increased, the greater is the number of beams among the primary beams 107, passing through tracks off the central axis (the optical axis) of the electron optical system, respectively, that is, the number of the beams passing through off-axial trajectories, respectively, will increase, so that effects of off-axial aberrations will become non-negligible. The off-axial trajectory refers to a track departing from a position on an object plane of the electron optical system, away from the optical axis, and reaching a position on an image face, away from the optical axis, and the off-axial aberration is an aberration caused by the charged particle beams passing through the off-axial trajectories, respectively. Since the aberration refers to magnitude of deviation causing a charged particle beam to fail to pass through an ideal position on an imaging plane, the charged particle beam, if affected by the off-axial aberration, cannot be focused in a stream over a specimen, coming to have a spread, so that resolution deterioration will result. The further away an off-axial distance from the optical axis, the greater is the effect of the off-axial aberration, so that it is necessary to compensate for the off-axial aberration in order to get around the trade-off between the inspection rate, and the resolution.
Further, as another approach, there has been proposed a projection charged particle beam apparatus wherein a primary beam is applied to a wide area of a specimen without being converged in a stream over the specimen, and secondary beam signals are projected on a detector with the use of an electron lens. With a projection inspection apparatus, images in block can be acquired without execution of scanning with the primary beams, so that it is possible to conduct inspection at a high rate. For example, in JP-A-Hei07 (1995)-249393, there has been disclosed a projection inspection apparatus wherein image formation is effected from backscattered electrons, and secondary electrons with the use of an electron lens, and in JP-A-Hei11 (1999)-108864, there has been disclosed a projection inspection apparatus for detecting electrons pulled back by a reversed electric field directly above the specimen before collision with a specimen, that is, mirror electrons. With the projection apparatus, it is necessary to widen an area of a specimen to be irradiated at a time in order to improve an inspection rate. The wider the area is, the greater is the number of the beams passing through the off-axial trajectories, so that the off-axial aberrations will pose a problem even with the projection apparatus.
Thus, either with the multi-beam type charged particle beam apparatus, or with the projection charged particle beam apparatus, there is the need for using the beams passing through the off-axial trajectories, away from the center of the electron optical system, respectively, so that it is necessary to compensate for the off-axial aberration in order to get around the trade-off between the inspection rate, and the resolution.
In the respective fields of a transmission electron microscope and a scanning electron microscope, an aberration corrector has become operational, and image observation higher in resolution has since come close to practical use by correction of axial chromatic aberration and spherical aberration. For example, in JP-A-Hei05 (1993)-205687, and JP-T-2004-519084, there has been disclosed an aberration correction means for correcting aberration by taking advantage of a feature of an electrostatic mirror in that aperture aberration as well as chromatic aberration thereof becomes negative.
Furthermore, there has also been proposed an aberration corrector for correcting not only axial aberration such as the aperture aberration, the axial chromatic aberration, and so forth, but also the off-axial aberration. For example, in JP-A-2003-187731, there has been disclosed off-axial aberration correction means wherein octupole components are overlapped on an electron optical system comprised of 2 systems of 7-piece set of quadrupole parts with symmetry maintained. Further, in JP-A-2007-109531, there has been proposed a charged particle beam apparatus provided with the off-axial aberration correction means as proposed in JP-A-2003-187731.